Investigate and determine which retinoids are highly effective in inhibiting N-nitrosomethylurea (NMU) induced mammary carcinogenesis. Determine the time interval which can elapse between the carcinogenic insult and initiation of retinoid treatment without impairing the efficacy of the retinoid in inhibition of mammary carcinogenesis. Evaluate the following parameters in assessing the anti-carcinogenesis activity of each retinoid studies: percent incidence mammary cancer and benign mammary tumors; number of mammary cancers and benign tumors; number of invasive tumors; number of mitotic figures; and latent period of tumor appearance. Observe experimental animals daily for evidence of retinoid toxicity. Evaluate all retinoids tested for evidence of toxicity using liver function and histological measures. Establish periodicity of the estrus cych by vaginal smears of rats in the groups. Determine serum levels of retinoids on a bi-monthly basis for each retinoid investigated.